Pyrrole-like defects as origin of piezoelectric effect in nitrogen-doped carbon nanotubes

In this work, it is shown for the first time that nitrogen-doped carbon nanotubes (CNTs) exhibit anomalous piezoelectric properties that depend on the concentration of pyrrolic N, which can be used in the development of energy-efficient nanogenerators. The results are supported by studies of CNTs using X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy, and piezoresponse force microscopy. We proposed a mechanism for appearance of a piezoelectric response in CNTs associated with the formation of an uncompensated dipole moment as a result of curvature of the graphene sheet surface due to the formation of pyrrole-like defects and bamboo-like “bridges” in the nanotube cavity. It was found that with an increase in the concentration of pyrrolic N from 20 to 27 at. %, the magnitude of the piezoelectric strain coefficient of CNTs increases from 30 to 92 pm/V, and the magnitude of the current generated during the deformation of the CNTs increases from 16 to 129 nA. Experimental and theoretical estimates of the output voltage arising at 1% deformation of nitrogen-doped CNTs are performed. The output voltage of CNTs ranged from 85 to 259 mV, depending on the pyrrolic N concentration. The established regularities make it possible to take into account the influence of the CNT defect types on its electrical properties in the process of creating various nanoelectronic devices.